Battery chargers which are used to charge two-way radio batteries typically charge the rechargeable battery at a fast charge rate for a certain period of time and then revert to a slower charge rate (trickle charge rate) for as long as the battery remains in the charger. The charger reverts to the trickle charge rate after determining that the change in temperature of the battery has reached a certain change in temperature over a predetermined period of time (.DELTA.T). This is done to protect the battery from overheating, since the battery at this specific point is almost fully charged. A second method used by prior art chargers to decide when to revert from the fast charge rate to the trickle rate mode is by monitoring the voltage of the battery and reverting to that mode upon a predetermined voltage slope is reached (.DELTA.V).
Referring now to FIG. 1, there is shown a prior art battery charging routine. In step 102, the battery is inserted into the charger. The charger in step 104 then determines the charge capacity of the battery by sensing the resistance of a capacity resistor (R.sub.c) corresponding to the capacity of the battery as is well known in the art. The capacity resistor (R.sub.c) is located in the battery itself. Also, the temperature of the battery is sensed by sensing the resistance (by determining the voltage developed by the resistance) of a thermistor which is also located inside of the battery. If the measurements taken in step 104 are determined to be satisfactory, the charge sequence is enabled. If the values measured are not within predetermined parameters, the charger goes to step 108, where the charger indicates a fault condition. The charger user is informed of the fault condition by lighting up a fault condition light emitting diode (LED) located on the charger.
In step 110, the rapid charge sequence is begun. In step 112, the charger monitors the .DELTA.T (temperature slope) of the battery to determine if the rapid charge rate is to be discontinued. Instead of monitoring for changes in .DELTA.T, the charger could monitor for changes in .DELTA.V, waiting until the slope becomes negative for .DELTA.V, which indicates that the battery is charged. If the .DELTA.T of the battery as determined by the charger reaches a predetermined value, the rapid charge sequence is stopped, in order to prevent overheating of the battery. The charger is then placed in a trickle charge mode in step 114. In the trickle mode the battery is charged at a rate of approximately C/10 to C/20, were "C" is the capacity of the battery. For example, if the battery has a capacity of 1000 milliampere-hour (maH) at a C/10 charge rate, the charger would charge the battery using a current of 100 ma.
Prior art chargers have a problem with users who leave their radios on and attached to the batteries, when the batteries are being charged. This is especially true for users who use the radio/charger combination as a desk top base station and transmit with the radio in the charger. In these particular cases, the amount of charge going into the battery in the trickle charge mode is less than the amount of charge being used by the radio in the stand-by mode. The battery charge depletion rate worsens if the radio begins to receive a message, or if the radio is used to transmit while still in the charger. This causes problems for users who think that their batteries are charged after being in the charger for a predetermined period of time, when in reality, in some conditions, the batteries are more depleted than when they were first inserted into the charger. In worst case scenarios, if the batteries are left in the charger long enough under the conditions previously mentioned, the batteries can become completely discharged, while the charger is still identifying to the user that the battery is fully charged.
Table 1 below shows the length of time under different conditions in which a battery will become completely discharged even while in a charger. Table 1 assumes that during a radio transmit condition, the radio which is attached to the battery under charge draws 2.1 amperes, during receive the current draw is 210 milliamps, and during standby the current draw is 70 milliamps. Part (a) assumes the radio is transmitting for 5% of the time, 5% it is receiving, and 90% it is in the standby mode. Parts (b) and (c) assume different ratios. As can be seen from the data presented in table 1, even after the charger has completed its rapid charge mode which places the battery in an almost fully charged condition, if a radio which is turned on is attached to the battery, charge depletion of the battery occurs. For example, if the battery has a 1200 maH capacity, and a 5/5/90 operational duty cycle is assumed for the radio, in approximately 9.6 hours the battery will be fully depleted, even given the trickle charge rate of 55 ma.
TABLE 1 ______________________________________ Capacity trickle rate net current Time ______________________________________ (a). 5/5/90 duty cycle load = 180 mA 900 MAH 55 mA (125 ma) 7.2 hrs 1200 MAH 55 mA (125 mA) 9.6 hrs 1800 MAH 90 mA (90 mA) 20 hrs (b). 10/10/80 duty cycle load = 287 mA 900 MAH 55 mA (232 ma) 3.9 hrs 1200 MAH 55 mA (232 mA) 5.2 hrs 1800 MAH 90 mA (197 mA) 9.1 hrs (c). 0/20/80 duty cycle load = 98 mA 900 MAH 55 mA (43 ma) 20.9 hrs 1200 MAH 55 mA (43 mA) 27.9 hrs 1800 MAH 90 mA (8 mA) 9.4 days ______________________________________
A need thus exists for a battery charger which can maintain the battery within a predetermined charge level even if the battery charge is being depleted as mentioned previously. Also, a need exists for a method of charging a battery which provides for better charging of the battery based on the individual characteristics of the battery being charged, thereby helping to extend the useful life of the battery.